The present invention relates to an orthogonal frequency division multiplexing (OFDM) system, and particularly, to a fast Fourier transform (FFT) window position recovery apparatus and method in an OFDM receiver.
FIG. 1 is a block diagram showing the configuration of a general OFDM system receiver. First, a symbol of an OFDM signal, when N subcarriers are used in the OFDM system, comprises N useful data samples output by transmitting inverse fast Fourier transform (IFFT) and a guard interval having the lengths of G samples inserted before a useful data interval to prevent interference between the symbols. Here, the guard interval copies the end portion of the useful data interval. A transmitter (not shown) adds G complex values to N complex values output by an inverse fast Fourier transformer (IFFT), and sequentially transmits a symbol comprised of a total of (G+N) samples. Here, the guard interval is generally longer than a delay spread time of a channel. For example, a European digital TV broadcast standard defines guard intervals having lengths of {fraction (1/4, 1/8, 1/16)} and {fraction (1/32)} of an actual symbol length, and a transmission side selects and uses one among the above-described guard intervals. Time synchronization must be accurately performed to allow a receiver to recover a received OFDM signal. Time synchronization is comprised of FFT window position recovery for accurate parallel processing of a signal, and sampling clock recovery for sampling a signal having a maximum signal-to-noise ratio (SNR). A j-th symbol comprised of a useful interval and a guard interval which are output by an IFFT (not shown) of a transmitter is expressed by the following Equation 1:                               s          j                =                                            ∑                              n                =                                  -                  G                                                            N                -                1                                      ⁢                          x                              j                ,                n                                              =                                                    ∑                                  n                  =                                      -                    G                                                                    -                  1                                            ⁢                                                ∑                                      k                    =                    0                                                        N                    -                    1                                                  ⁢                                                      X                                          j                      ,                      k                                                        ⁢                                      ⅇ                                          j                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              xe2x80x83                                            ⁢                                                                        π                          ⁡                                                      (                                                          N                              +                              n                                                        )                                                                          /                                                                              ⁢                  N                                                      +                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                ∑                                      k                    =                    0                                                        N                    -                    1                                                  ⁢                                                      X                                          j                      ,                      k                                                        ⁢                                      ⅇ                                          j2                      ⁢                                              xe2x80x83                                            ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      k                      ⁢                                              xe2x80x83                                            ⁢                                              n                        /                        N                                                                                                                                                    (        1        )            
wherein j denotes a symbol number, k is a carrier index, N is the number of effective data samples, and n indicates a sampling time. In the right side of Equation 1, the first term is a guard interval portion, and the second term is a useful data portion.
As shown in FIG. 1, an analog-to-digital converter (ADC) 110 samples a received OFDM signal. A symbol start detector 120 receives the sampled OFDM signal and detects a start portion of a symbol. A FFT window controller 130 designates a FFT window point in time to activate a useful data portion of a FFT 140 using the symbol start information detected by the symbol start detector 120. As shown in FIG. 2, the FFT window transmits only the useful data interval excluding the guard interval in a received signal to the FFT. Generally, the symbol start detector 120 of FIG. 1 detects a symbol start portion using either a correlation value between received signals or the quantized values of the input signals. However, the first way requires a maximum position of the correlation value between the received signals, resulting in complicated system realization. The second way provides a simple structure because of the 2-bit quantization. However, when a frequency offset defined as a carrier frequency synchronization error, i.e., a phase difference of ej(xcfx890t+xcfx86), exists, phase rotation of the input signal occurs to thus change the phases of a guard interval and the end portion of a useful data interval. Thus, a position having a maximum correlation value cannot be found, and symbol starting position recovery is thus difficult.
It is an object of the present invention to provide a method for recovering a phase error of a FFT window using a power difference between received signals in a predetermined section in an orthogonal frequency division multiplexer (OFDM) receiver.
It is another object of the present invention to provide an apparatus for recovering a phase error of a FFT window using a power difference between received signals in a predetermined section in an OFDM receiver.
To accomplish the first object, there is provided a fast Fourier transform (FFT) window position recovery method in an orthogonal frequency division multiplexing (OFDM) system receiver which receives an OFDM symbol comprised of a useful data interval and a guard interval and recovers a FFT window position, the method comprising the steps of: (a) converting a received OFDM signal into a digital complex sample; (b) obtaining a power difference between the digital complex sample output by the step (a) and a complex sample value delayed by predetermined samples, and detecting a position having a maximum power value between the samples, as a symbol starting position; and (c) activating FFT while moving the FFT window position using symbol start information detected by the step (b).
To accomplish the second object, there is provided a fast Fourier transform (FFT) window position recovery apparatus in an orthogonal frequency division multiplexing (OFDM) system receiver which receives an OFDM symbol comprised of a useful data interval and a guard interval and recovers a FFT window position, the apparatus comprising: an analog-to-digital converter (ADC) for converting a received OFDM signal into a digital complex sample; a symbol start detector for detecting a power difference between the digital complex samples output by the ADC and detecting a position having a minimum absolute value of the power difference between the samples, as a symbol starting position; and a FFT window controller for activating FFT by moving the FFT window position using symbol start information detected by the symbol start detector.